A navigation device for a vehicle generally has route guidance function which executes route guidance for a driver of the vehicle when the vehicle approaches a point of guidance such as an intersection to turn, a diverting point, an entrance to a highway or an exit from the highway. In the route guidance, the navigation device causes a display device to display a magnified view of a map including the point of guidance and causes a sound device to output a voice announcing a route to take at the point of guidance such as “Turn left at a point xxx meters ahead.”
In JP 2003-262530A, a navigation device is disclosed which is intended for executing the route guidance at a suitable timing. More specifically, this navigation device recursively estimates a period required to reach an intersection and gives the route guidance when the estimated period becomes lower than a predetermined period. This navigation device thus can initiate the route guidance while giving the driver the same time margin before reaching every point of guidance.
In addition, this navigation device changes a timing to execute the route guidance and a manner of executing the route guidance based on a condition such as driving skill of the driver, weather and brightness around the vehicle. This navigation device thus can execute the route guidance based on the driving skill of the driver or a condition surrounding the vehicle.
A degree of traveling stability of the vehicle changes depending on a condition of a road to travel and a condition of the vehicle. In the case that the tires of the vehicle are slippery, the tires tend to lock or skid and the degree of the traveling stability of the vehicle accordingly decreases. On an uphill road and a downhill road, loads on the tires become uneven. One of the tires under a lighter load is more likely to lock or spin out. It is desirable to avoid a driving operation such as a sudden braking or a sudden lane change when the degree of the traveling stability of the vehicle is low.
The point of guidance is a point at which the vehicle has to change its heading direction. The vehicle hence has to change a lane to travel on or reduce its speed before the point of guidance. The degree of the traveling stability of the vehicle may therefore become so low that the driver is given a time margin insufficient for changing the lane or reducing the speed after the route guidance made by the conventional navigation device.